1. Field of Invention
The present invention relates to a method of coating the surface of an inorganic powder and a coated inorganic powder manufactured by this method, more particularly, it is related to a method of depositing a uniform metal oxide coating on the surface of an inorganic powder which can be used in the manufacture of Multi Layer Ceramic Capacitor or as an active material in lithium batteries.
2. Description of the Related Art
Multi Layer Ceramic Capacitor (hereinafter ‘MLCC’) consists of multi-layers of capacitors in which a dielectric ceramic layer of BaTiO3 is inserted between thin-layer metal electrodes like Nickel or Copper. MLCC is widely used in computers, mobile communication equipments, and other small electronic equipments due to its small volume but large capacity.
Ag—Pd alloys that have been used as the metal electrode have the advantage of being able to be sintered in air, but it has the drawback of high manufacturing cost. Therefore, in the late 1990s in order to reduce the manufacturing cost, Ag—Pd alloys were replaced by Ni, and a Ni-MLCC technology of firing Ni was introduced in a reducing atmosphere to prevent oxidation of Ni.
FIG. 1 is a plan view illustrating schematically BaTiO3 powder dielectric layer 100 and Ni powder electrode layer pattern 200 formed during the manufacturing process of the Ni-MLCC.
The conventional manufacturing process of Ni-MLCC is described while referring to FIG. 1. First, BaTiO3 powder dielectric layer 100 is formed by coating a surface, such as a PET film, with BaTiO3 powder dispersion. Afterwards, the Ni powder dispersion is screen-printed on the BaTiO3 powder dielectric layer to form a plurality of Ni powder electrode layer pattern 200. By repeating the processes of forming BaTiO3 powder dielectric layer 100 and Ni powder electrode layer pattern 200, multi-layers of BaTiO3 powder layer 100 and Ni powder electrode layer pattern 200 are formed. This multilayer is then cut along the cutting line 300 and sintered to transform BaTiO3 powder dielectric layer 100 and Ni powder electrode layer pattern 200 into BaTiO3 mono-layer dielectric layer and Ni mono-layer electrode layer pattern, respectively. This completes the manufacturing of Ni-MLCC.
The Ni powder electrode layer pattern 200 contains a large amount of organic vehicle prior to the sintering process, which causes the Ni powder to have a relatively low packing density. Therefore, when sintering, the Ni powder electrode layer pattern 200 shows greater shrinkage than the BaTiO3 powder dielectric layer 100.
In addition, the sintering temperature of Ni powder is about 600° C. and that of BaTiO3 is about 1250˜1300° C. Ni powder starts to shrink significantly around 400˜500° C., while, BaTiO3 powder starts to shrink beyond about 1100° C. Therefore, in the sintering process, the Ni powder electrode layer pattern 200 starts to shrink at a temperature range of 400˜500° C. but the BaTiO3 powder dielectric layer 100 shows no actual shrinkage in this temperature range.
Due to the difference in the shrinkage rate and in the temperature range of shrinkage between the BaTiO3 powder dielectric layer 100 and the Ni powder electrode layer pattern 200, there is a strong contraction stress between the above two layers. From time to time, this contraction stress causes severe problems like poor contact ability between electrodes or the delamination of layers between the Ni electrode layer and the BaTiO3 dielectric layer.
Accordingly, several solutions have been proposed to solve the poor contact between electrodes or the above delamination of layers between the Ni electrode and the BaTiO3 dielectric layer. One solution to these problems, was to fill the pores among the Ni powders with BaTiO3 powder to reduce shrinkage of the Ni powder electrode layer pattern 200. However, this solution was not successful because the difference in shrinkage rate during sintering could not be decreased significantly since the entire surface of the Ni powders were not covered with the dielectric powders on purpose in order to secure contact between the Ni powders.
Another solution used to reduce heat-shrinkage rate of the Ni powder electrode layer is to form the Ni powder electrode layer pattern 200 by using Ni powder coated with a metal oxide, whose shrinkage starting temperature is close to that of BaTiO3. Metal oxides that can be used for coating the Ni powder are MgO, SiO2, TiO2, BaTiO3 and rare-earth metal oxides. These metal oxides can coat the surface of Ni powders using spray thermal decomposition or sol-gel coating process disclosed in U.S. Pat. No. 6,268,054, for example.
Spray thermal decomposition is a method of forming Ni powder in which a solution containing both thermally decomposable compounds and Ni powders are sprayed to a heating tube, and the thermally decomposable compounds are thermally decomposed, thereby producing Ni powders coated with metal oxide. However, using this method metal oxide is formed not only on the surface of the Ni powders but also within the Ni powders. This results in waste of the raw materials and also high processing costs.
In the sol-gel coating method, after dissolving coating materials in water, Ni powders are added in the solution and through the sol-gel reaction, the surface of the Ni powders are coated with the coating materials physically/chemically. The coated Ni powders are filtered, dried, and heat-treated, to thereby crystallize the coated layer. This method provides the Ni powders with a strong metal oxide coating layer, and allows mass production of coated Ni powders economically.
FIG. 2 is a flow chart illustrating the manufacture of Ni powders coated with titanium oxide by the sol-gel process. Referring to FIG. 2, an aqueous Ni slurry (1), i.e., Ni powder dispersed in water, is mixed with a TiCl4 aqueous solution and an NH4OH aqueous solution under stirring. TiCl4 precipitates as titanium hydroxides, Ti(OH)x, after undergoing reaction with hydroxide ions produced by the acid-base reaction formula illustrated below:NH4OH→NH4++OH−.
The resulting titanium hydroxide is deposited and coated on the surface of the Ni powders (3). The Ni powders coated with titanium hydroxide are then washed with alcohol (5). Washing with alcohol removes impurities and transforms the hydroxide ions of Ti(OH)x on the surface of the coated layer to an alkoxy group in order to reduce agglomeration of the Ni powders which may occur by condensation reaction among the hydroxide ions during the drying process. Afterwards, the coated Ni powders are dried (7). When drying is completed, the Ni powders are heat treated (9) at a temperature of 400˜500° C. in an oxidative atmosphere. In this heat treatment process, titanium hydroxide is transformed into titanium oxide TiO2. Thus, the manufacture of Ni powders coated with TiO2 is completed.
However, the conventional sol-gel coating method has a few drawbacks as summarized below because the method uses water as the coating medium. That is,
(1) Some of the coated Ni powders tend to become agglomerated. This is because of the formation of Ti—O—Ti bonding after condensation reaction between hydroxide ions of titanium hydroxide coated on the surface of a Ni powder and other hydroxide ions of titanium hydroxide coated on the surface of other Ni powders.
(2) A portion of titanium oxide may exist not on the surface of the Ni powders but as clusters between the spaces of the Ni powders and remain there, not coating the surface of the Ni powders. Further, a portion of the surface of Ni powders may remain uncoated and exposed. This is due to the usage of a large amount of water as the coating medium in which a large number of hydroxide ions are produced according to the acid-base reaction in a short period of time, and the hydroxide ions react with TiCl4 at a short period of time and produce large amount of titanium hydroxide precipitations. In this case, some of the titanium hydroxide precipitates are stabilized in the water medium and remain as a cluster before contacting the Ni powders to coat the surface of the Ni powders. Accordingly, some portions of the Ni powders remain uncoated with titanium oxide.
FIG. 3 is a SEM photograph of Ni powders that have been obtained by the conventional sol-gel coating method as illustrated by the flow chart in FIG. 2. Referring to FIG. 3, agglomerates of titanium hydroxide are seen between the coated spherical Ni powders, and a portion of the Ni powders is exposed without being coated with titanium hydroxide. These titanium hydroxide agglomerates are maintained during the heat treatment for crystallizing the coated layer and the agglomeration strength increases as the crystallization proceeds.
In the manufacturing of MLCC, forming a Ni electrode layer employing the Ni powders produced by the conventional sol-gel coating method causes following problems;
(1) When sintering, disconnection of Ni electrode increases. This is because Ni electrode layer, prepared by using the agglomerated Ni powders has a surface with increased roughness, increasing non-uniformity in the thickness of the Ni electrode layer.
(2) Delamination increases between the dielectric layer and the Ni electrode layer due to the large difference in the shrinkage temperature between the coated Ni powders and BaTiO3. The difference arises from the fact that the Ni powders are not coated uniformly and a portion of the surface remains uncoated and exposed.
For the above stated reason, the electrode layer produced using the Ni powders produced by the conventional sol-gel coating method degrades quality of MLCC and increases failure rate. Therefore, there is a need to develop a method of coating the surface of Ni powders with metal oxide without substantially forming agglomerates and with uniform thickness to improve of the quality of MLCC. A development of a method of coating inorganic powders without substantially forming agglomerates is also important in producing high capacity lithium batteries.